Magnetic Tunnel Junction with Low Defect Rate after High Temperature Anneal for Magnetic Device Applications

ABSTRACT

A magnetic tunnel junction is disclosed wherein the reference layer and free layer each comprise one layer having a boron content from 25 to 50 atomic %, and an adjoining second layer with a boron content from 1 to 20 atomic %. One of the first and second layers in each of the free layer and reference layer contacts the tunnel barrier. Each boron containing layer has a thickness of 1 to 10 Angstroms and may include one or more B layers and one or more Co, Fe, CoFe, or CoFeB layers. As a result, migration of non-magnetic metals along crystalline boundaries to the tunnel barrier is prevented, and the MTJ has a low defect count of around 10 ppm while maintaining an acceptable TMR ratio following annealing to temperatures of about 400° C. The boron containing layers are selected from CoB, FeB, CoFeB and alloys thereof including CoFeNiB.

PRIORITY DATA

The present application is a divisional application of U.S. patentapplication Ser. No. 15/835,592, filed on Dec. 8, 2017, which is adivisional application of U.S. patent application Ser. No. 14/803,111,filed on Jul. 20, 2015, now U.S. Pat. No. 9,842,988, each of which isherein incorporated by reference in its entirety.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. Nos. 8,059,374;and 8,946,834; both assigned to a common assignee, and which are hereinincorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to magnetic random access memory (MRAM),spin-torque MRAM, and other spintronic devices having a magnetic tunneljunction (MTJ) wherein magnetic layers are designed to prevent diffusionof non-magnetic elements into a tunnel barrier layer between twomagnetic layers thereby providing a low defect rate after hightemperature annealing around 400° C.

BACKGROUND

A MTJ is a key component in MRAM, spin-torque MRAM, and other spintronicdevices and comprises a tunnel barrier layer such as a metal oxideformed between two magnetic layers that together generate a tunnelingmagnetoresistance (TMR) effect. One of the magnetic layers is a freelayer and serves as a sensing layer by switching the direction of itsmagnetic moment in response to external fields (media field) while thesecond magnetic layer has a magnetic moment that is fixed and functionsas a reference layer. The electrical resistance through the tunnelbarrier layer (insulator layer) varies with the relative orientation ofthe free layer moment compared with the reference layer moment andthereby provides an electrical signal that is representative of amagnetic state in the free layer. In a MRAM, the MTJ is formed between atop conductor and bottom conductor. When a current is passed through theMTJ, a lower resistance is detected when the magnetization directions ofthe free and reference layers are in a parallel state (“0” memory state)and a higher resistance is noted when they are in an anti-parallel stateor “1” memory state. The tunnel barrier is typically about 10 Angstromsthick so that a current through the tunnel barrier can be established bya quantum mechanical tunneling of conduction electrons.

Both of the reference layer and free layer may have a syntheticanti-ferromagnetic (SyAF) configuration in which an outer layer isanti-ferromagnetically coupled through a non-magnetic coupling layer toan inner layer that contacts the tunnel barrier. MgO is often preferredas the tunnel barrier and provides a high TMR ratio when adjoining aCoFe or Fe inner magnetic layer. The TMR ratio is known as dR/R where Ris the minimum resistance of the MTJ, and dR is the change in resistanceobserved by changing the magnetic state of the free layer. A higher TMRratio improves the readout speed. Moreover, a high performance MTJrequires a low areal resistance RA (area×resistance) value of about 1ohm-um², a free layer with low magnetostriction (λ) between −5×10⁻⁶ and5×10⁻⁶, low coercivity (Hc), and low interlayer coupling (Hin) betweenthe free layer and reference layer through the tunnel barrier layer.

A high TMR ratio is obtained when the MTJ stack forms a face centeredcubic (fcc) crystal structure. However, naturally deposited CoFe or Fetends to form a body centered cubic (bcc) crystal orientation thatprevents formation of the fcc structure for a CoFe/MgO/CoFe stack. Apopular solution to this problem is to deposit amorphous CoFeB or FeBrather than CoFe or Fe. As a result, there is no template for crystalstructure growth until annealing when B tends to diffuse away from thetunnel barrier to leave a CoFe or Fe interface with the metal oxidetunnel barrier. Meanwhile, MgO forms a fcc structure that induces fcccrystal growth in the adjoining magnetic layers. The boron content inthe CoFeB and FeB layers is about 20% or less since B needs to diffuseaway from the interfaces with the tunnel barrier layer to achieve highTMR ratio.

In order to realize a smaller Hc but still maintain a high TMR ratio,the industry tends to use CoFeB as the free layer in a TMR sensor.Unfortunately, the magnetostriction (λ) of a CoFeB free layer isconsiderably greater than the maximum acceptable value of about 5×10⁻⁶for high density memory applications. Furthermore, a free layer mayinclude a non-magnetic insertion layer (INS) in a FL1/INS/FL2 stack, forexample, where the insertion layer is sandwiched between twoferromagnetic layers (FL1, FL2) to provide anti-ferromagnetic couplingor a moment diluting effect. However, the non-magnetic materials do notbind well with CoFeB and tend to diffuse at elevated temperatures alonggrain boundaries in crystalline magnetic layers. As a result,non-magnetic metals may diffuse through a free layer or reference layerinto the tunnel barrier during or after annealing, and disrupt theinsulation property of the barrier thereby causing a device defect. Thistype of diffusion is even more pronounced in semiconductor deviceswherein MRAM devices are integrated (embedded) with complementarymetal-oxide-semiconductor (CMOS) units that are processed attemperatures as high as 400° C. Thus, an improved free layer (orreference layer) design is needed to reduce non-magnetic metal diffusioninto a tunnel barrier layer while maintaining other MTJ properties suchas low λ, Hc, and Hin, and high TMR ratio following process temperaturesas high as 400° C.

SUMMARY

One objective of the present disclosure is to provide a magnetic layerstructure that can serve as one or both of a free layer or referencelayer in a MTJ, and maintain a defect level of <50 ppm therein afterhigh temperature annealing by inhibiting the migration of non-magneticmetals toward an interface with an adjoining tunnel barrier.

A second objective of the present disclosure is to provide a MTJ with amagnetic layer structure according to the first objective that also hasacceptable λ, Hc, Hin, and TMR ratio values.

A further objective is to provide a method of forming a MTJ with amagnetic layer structure according to the first and second objectivesthat can be readily implemented in a manufacturing process and is costeffective.

According to one embodiment of the present disclosure, these objectivesare achieved with a bottom spin valve configuration when forming a MTJon a suitable substrate such as a bottom conductor in a MRAM device. Aseed layer and an optional antiferromagnetic (AFM) pinning layer may besequentially formed on the bottom conductor. In a preferred embodiment,a reference layer/tunnel barrier/free layer stack of layers is formed onthe seed layer or AFM layer. The reference layer may have an AP2/NM1/AP1configuration wherein an “outer” AP2 ferromagnetic layer contacts theseed layer or AFM layer, NM1 is a first non-magnetic metal or alloylayer used for an antiferromagnetic coupling or moment diluting effect,and AP1 is an inner ferromagnetic layer that contacts the tunnel barrierat a first surface. AP1 comprises a first layer with a B content of25-50 atomic %, and a second layer having a B content from about 1-20atomic %. The first and second AP1 layers form an interface with eachother, and one of the two AP1 layers contacts the tunnel barrier. Theremay be a third AP1 layer that is formed a greater distance from thetunnel barrier than the first and second layers such that the third AP1layer adjoins the NM1 layer. The tunnel barrier is preferably MgOalthough other metal oxides, metal nitrides, or metal oxynitrides may beused. The free layer stack has first magnetic layer with a high Bcontent of 25-50 atomic %, and a second magnetic layer with a B contentfrom 1-20 atomic %. The first and second magnetic layers form aninterface with each other and one of the two magnetic layers contactsthe tunnel barrier on a surface that is opposite the first surface.

In some embodiments, the free layer stack has a FL1/FL2/NM2/FL3 whereFL1 and FL2 are the first and second magnetic layers, NM2 is a secondnon-magnetic metal or alloy, and FL3 is a third magnetic layer. The Bcontaining layers with elevated B content in the AP1 reference layer andfree layer stack are advantageously employed to substantially reduce themigration of non-magnetic metals in NM1 and NM2, respectively, to thetunnel barrier.

The FL1, FL2, and first and second AP1 layers have a composition that isselected from CoB, FeB, CoFeB, CoFeNiB, or CoFeBQ where Q is one of Zr,Hf, Nb, Mo, Ta, and W. In some embodiments, one or more of FL2 and theAP1 layers may be made of a different alloy than the FL1 alloy. Forexample, FL1 may be CoB while one or more of FL2 and the AP1 layers areone of FeB, CoFeB, CoFeNiB, or CoFeBQ. Each of the B containing layershas a thickness of 1 to 10 Angstroms. In some embodiments, a boroncontaining layer such as CoFeB may comprise a bilayer that is CoFe/B orB/CoFe. In other embodiments, the boron containing layer may be amultilayer structure with one or more B layers and one or more layers ofCo, Fe, CoFe, CoFeB, CoFeNi, or CoFeQ. NM1 and NM2 are selected from ametal M that is Ru, Rh, Ir, Ta, W, Mo, Cr, and Mg, or may be an alloyincluding one of the M metals and one of Ni, Fe, or Co. Preferably, theFL1/FL2 stack and the AP1 layer each have a total thickness less thanabout 20 Angstroms to promote perpendicular magnetic anisotropy in thefree layer and reference layer, respectively.

In an alternative embodiment, the MTJ may have a top spin valvestructure where the free layer, tunnel barrier, and reference layer aresequentially formed on the seed layer or AFM layer. In this case, thefree layer has a FL3/NM2/FL2/FL1 stack where FL3 contacts the seed layeror AFM layer, and FL1 contacts a bottom surface of the tunnel barrier.The reference layer may have an AP1/NM1/AP2 configuration wherein theAP1 layer contacts a top surface of the tunnel barrier.

All of the MTJ layers may be deposited in the same sputter chamber.Magnetic layers may be deposited at room temperature or up to 400° C.The tunnel barrier is typically formed by first depositing a metal layersuch as Mg on the AP1 reference layer, performing a natural oxidation orradical oxidation step, and then depositing a second metal layer on theoxidized first metal layer. The metal deposition and oxidation sequencemay be repeated before an upper metal layer is deposited on the tunnelbarrier stack. After all layers in the MTJ are laid down, a hightemperature anneal up to 400° C. is performed to transform the tunnelbarrier stack into a substantially uniform metal oxide layer wherein theupper metal layer becomes oxidized. Thereafter, the MTJ stack ispatterned to form a plurality of MTJ elements. A dielectric layer isdeposited to fill the gaps between adjacent MTJ elements prior toforming a top electrode thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1b are cross-sectional views showing a MTJ stack of layers in abottom spin valve configuration according to an embodiment of thepresent disclosure.

FIGS. 1c-1d are cross-sectional views showing a MTJ stack of layers in abottom spin valve configuration wherein each of the AP1 layers in thereference layer has a bilayer or trilayer configuration, respectively,according to an embodiment of the present disclosure.

FIGS. 1e-1f are cross-sectional views showing a MTJ stack of layers in abottom spin valve configuration wherein each of the FL1 and FL2 layersin a FL1/FL2 free layer stack has a bilayer or trilayer configuration,respectively, according to an embodiment of the present disclosure.

FIGS. 2a-2b are cross-sectional views that depict a MTJ stack of layersin a top spin valve configuration according to another embodiment of thepresent disclosure.

FIG. 3 is a cross-sectional view of a MTJ stack of layers with a bottomspin valve configuration according to a third embodiment of the presentdisclosure.

FIG. 4 is a cross-sectional view of a MTJ stack of layers with a topspin valve configuration according to a fourth embodiment of the presentdisclosure.

FIG. 5 is a cross-sectional view of a MTJ stack having a bottom spinvalve configuration formed according to a fifth embodiment of thepresent disclosure.

FIG. 6 is a cross-sectional view of a MTJ stack having a bottom spinvalve configuration formed according to a sixth embodiment of thepresent disclosure.

FIGS. 7-9 depict cross-sectional views and show a sequence of steps forforming a MRAM device that comprises a MTJ element according to anembodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to a MTJ element wherein each of areference layer and free layer are multilayer structures including afirst layer with high B content of 25-50 atomic %, and a second layerwith a low boron content from 1-20 atomic %. The high B content layersprevent non-magnetic metals in other portions of the MTJ from migratinginto the tunnel barrier during annealing and other processes withtemperatures proximate to 400° C. thereby enabling devices with lowdefect counts around 10 ppm. The MTJ may have a bottom spin valve, topspin valve, or dual spin value configuration as appreciated by thoseskilled in the art. The MTJ element may be implemented in a variety ofmemory devices including but not limited to MRAM, embedded MRAM,spin-torque MRAM, and other spintronic devices such as a spin torqueoscillator (STO).

As mentioned previously, many memory devices including embedded MRAM arenow incorporated into CMOS platforms to provide higher performance.However, we have observed a substantially higher defect rate whenconventional MTJ elements annealed in the range of 300-330° C. aresubsequently exposed to temperatures around 400° C. that are required inCMOS processing. Thus, we were motivated to redesign the typicalreference layer/tunnel barrier/free layer stack in a MTJ to becompatible with CMOS fabrication by modifying each of the free layer andreference layer to enable a low defect rate of <50 ppm (defects permillion parts), and preferably about 10 ppm, after 400° C. annealing.

Although not bound by theory, it is our belief that a means ofpreventing non-metal migration into a tunnel barrier layer is to disruptcrystal formation in at least a portion of the free layer and/orreference layer that is proximate to the tunnel barrier. Non-magneticmetals such as Ru, Rh, or Ir that are employed as antiferromagneticcoupling agents in a reference layer, or Ta, Mo, W, Mg, Cr, and the likethat are used for a moment diluting effect in a free layer do not bindwell with magnetic layers including CoFeB or the like. Thus, when CoFeBwith low B content below 20 atomic % begins to crystallize at annealingtemperatures between 300° C. and 400° C., pathways are created at grainboundaries and become channels for non-magnetic metal migration fromwithin the free layer or reference layer to the tunnel barrier. We havediscovered that by increasing the amorphous character of a referencelayer and free layer in a portion thereof proximate to the tunnelbarrier, crystal formation in said region is disrupted or delayed to anextent that considerably slows movement of non-magnetic metals to thetunnel barrier. As a result, low defect levels that are 10 ppm, forexample, after conventional annealing at 330° C. may also be achievedafter elevated annealing temperatures of about 400° C. Here, the term“about 400° C.” is defined to mean temperatures that may in someembodiments reach 410-420° C. for 30 minutes or less.

Referring to FIGS. 1a-1b , a first embodiment of the present disclosureis shown wherein a MTJ stack 1 is depicted from a cross-sectional view.The MTJ stack of layers in a bottom spin valve configuration may beformed on a bottom conductor (not shown) in a MRAM device, for example.The bottom layer 10 in the MTJ stack may be a seed layer such as NiCr,NiFeCr, or other materials used in the art to promote the desiredcrystal structure in overlying layers. In other embodiments, the bottomlayer may be a stack including a plurality of seed layers selected fromTa, Ru, NiCr, Cr, NiFeCr, Zr, Hf, Nb, Mg, Ti, and the like. An optionalantiferromagnetic (AFM) pinning layer 34 that is one of IrMn, PtMn,NiMn, OsMn, RuMn, RhMn, PdMn, RuRhMn, or MnPtPd may be employed betweenthe bottom layer and the AP2 magnetic layer 11 in order to pin themagnetic moment in the overlying AP2 magnetic layer. The AP2 magneticlayer is part of a reference layer 15 a 1 (FIG. 1a ) or 15 b 1 (FIG. 1b) having an AP2/NM1/AP1 configuration where NM1 is a first non-magneticlayer.

When the optional AFM layer is inserted between the bottom layer 10 andthe AP2 layer 11, the AP2 layer may also be referred to as a pinnedlayer having a magnetic moment 11 a that is fixed an in-plane direction(FIG. 1a ). In an alternative embodiment where the magnetic moment 11 bis fixed in a perpendicular-to-plane direction (FIG. 1b ), an AFM layeris not required. In FIG. 1b , the AP2 layer is said to haveperpendicular magnetic anisotropy (PMA) where the shape anisotropy thatpromotes magnetization in an in-plane direction is less than themagnitude of the PMA component. Preferably, when the AP2 layer has PMA,the AP1 layer 17 that is comprised of lower magnetic layer 13 and uppermagnetic layer 14 also has PMA. The AP1 layer may further comprise anoptional magnetic layer 16 made of Co, Fe, CoFe, or alloys thereof withNi, B, or other metals that is understood to have a magnetic moment inthe same direction as layers 13, 14.

In FIG. 1a where the NM1 layer 12 is an antiferromagnetic coupling layersuch as Ru, Rh, or Ir, AP1 layers 13, 14 have a magnetic moment 13 a, 14a, respectively, in a direction opposite to that of magnetic moment 11 ain the AP2 layer. Thus, the reference layer is said to have a syntheticanti-parallel (SyAP) configuration that is beneficial in balancing thebipolar field and writing symmetry. In one aspect, at least the upperAP1 layer 14 is amorphous when deposited in order to provide a moreuniform surface on which to form the tunnel barrier 20.

In a preferred embodiment shown in FIG. 1b , AP1 layers 13, 14 have amagnetic moment 13 b, 14 b, respectively, opposite to the magneticmoment 11 b for the AP2 layer, and all AP1, AP2 layers have PMA. The PMAconfiguration in FIG. 1b is typically favored over the in-planeembodiment since PMA provides higher thermal stability for referencelayer 15 b 1 (and free layer 35-1) as the MTJ in-plane dimensions arescaled down to provide higher density memory devices.

In alternative embodiments (not shown) where the NM1 layer 12 is amoment diluting layer that is a made of an element M selected from Ru,Ta, Ti, W, Zr, Hf, Nb, Mo, V, Mg, and Cr, or is an alloy which includesa magnetic element (Fe, Co, or Ni) and a non-magnetic element M, themagnetic moments of all three layers 11, 13, 14 are aligned in the samedirection. The NM1 layer is preferably amorphous to block the growth ofa crystalline AP1 layer until a subsequent anneal step. In someembodiments, all magnetic moments are in-plane while in otherembodiments where all AP1 and AP2 layers have PMA, all magnetic momentsin the reference layer 15 b 1 are in a perpendicular-to-plane directionwhen NM1 is a moment diluting layer. A moment diluting material in thiscontext is defined as a non-magnetic metal or alloy that is employed toreplace a portion of the magnetic material in a reference (or free)layer thereby decreasing the overall magnetic moment for the referencelayer or free layer while maintaining essentially a constant referencelayer or free layer thickness. It should be understood that when NM1 isan alloy, increasing the content of the magnetic element in the alloywill increase the coupling strength between the AP1 and AP2 layers butmay lower the TMR ratio. Furthermore, the thickness of the NM1 layer mayvary from about 1 to 10 Angstroms in order to adjust the TMR ratio,magnetostriction (λ), and coupling strength (Hin) between the AP1 andAP2 layers.

Returning to FIG. 1a or FIG. 1b , the lower AP1 layer 13 is a firstboron containing alloy wherein the B content is from 25-50 atomic %, andthe upper AP1 layer 14 is a second boron containing alloy wherein the Bcontent is from 1 to 20 atomic %. In related U.S. Pat. Nos. 8,059,374and 8,946,834, we disclosed an upper limit to B content of 40 atomic %in a magnetic alloy in a magnetic layer. However, we have surprisinglydiscovered that the B content may be raised to 50 atomic % in the firstboron containing alloy in AP1 layer 13 and in the subsequently depositedFL1 layer 30 without substantially affecting MTJ performance includingTMR ratio. As mentioned earlier, boron tends to migrate away from thetunnel barrier during annealing to leave a region of free layer (andreference layer) adjacent to the tunnel barrier that is substantiallyfree of boron. A high TMR ratio is thereby achieved with the resultingreference layer/tunnel barrier/free layer stack.

The first and second boron containing layers are not necessarily formedfrom the same elements. Each of the AP1 layers 13, 14 has a compositionthat is selected from one of CoB, FeB, CoFeB, CoFeNiB, or CoFeBQ where Qis one of Zr, Hf, Nb, Ta, Mo, or W. Preferably, each of AP1 layer 13 andAP1 layer 14 has a thickness from 1 to 10 Angstroms. Furthermore, one orboth of the AP1 layers may be comprised of a bilayer configuration suchas 13-1/13-2 (and 14-1/14-2) as depicted in FIG. 1c wherein a firstlayer is Co, Fe, CoFe, CoFeNi, CoFeQ, or CoFeB, and the second layer isB. For example, instead of a CoFeB alloy, one or both of the AP1 layers13, 14 may be represented by a CoFe/B or B/CoFe configuration. In yetanother embodiment shown in FIG. 1d , one or both of the AP1 layers mayhave a multilayer configuration such as 13-1/13-2/13-3 (and14-1/14-2/14-3) including one or more boron layers, and one or morelayers selected from Co, Fe, CoFe, CoFeB, CoFeNi, and CoFeQ in order toadjust the boron content in an AP1 layer.

Tunnel barrier 20 contacts a top surface of AP1 layer 14. The tunnelbarrier may be an oxide, oxynitride, or nitride of Mg, Ti, AlTi, MgZn,Al, Zn, Zr, Ta, or Hf, or a native CoFeB, CoB, or FeB oxide. In otherembodiments, the tunnel barrier may be a laminated stack of one or moreof the aforementioned materials. The tunnel barrier is typically around10 Angstroms thick but the thickness may be adjusted to tune theresistance×area (RA) value. As the tunnel barrier thickness increases orthe degree of oxidation of the metal or alloy in the tunnel barrierincreases, the RA value also becomes greater.

In the most general embodiment, the free layer stack 35-1 in FIG. 1a andFIG. 1b has only two magnetic layers 30, 31 where a first free layer(FL1) 30 contacts a top surface of the tunnel barrier 20 and has a highboron content of 25-50 atomic %, and a second free layer (FL2) 31 has alow boron content from 1-20 atomic % and forms an interface with thefirst free layer. The FL1 and FL2 layers may have differentcompositions. However, both layers 30, 31 have a composition that isselected from CoB, FeB, CoFeB, CoFeNiB, or CoFeBQ described previously.Also, the present disclosure encompasses an embodiment depicted in FIG.1e wherein the alloy in one or both FL1, FL2 layers is replaced by abilayer 30-1/30-2 (and 31-1/31-2) that includes a boron layer, and alayer of Co, Fe, CoFe, CoFeNi, or CoFeQ. For example, a CoFeB alloylayer may be replaced by a bilayer represented by CoFe/B or B/CoFe.Moreover, one or both of FL1 and FL2 may have a multilayer configuration30-1/30-2/30-3 (or 31-1/31-2/31-3) comprised of one or more B layers andone or more layers selected from Co, Fe, CoFe, CoFeB, CoFeNi, and CoFeQas shown in FIG. 1f . Each of the FL1 and FL2 layers preferably has athickness from 1 to 10 Angstroms.

In some embodiments, optional layers 32, 33 are included. Secondnon-magnetic (NM2) layer 32 is one of Ru, Rh, or Ir and functions as ananti-ferromagnetic coupling layer thereby causing the magnetic moments(not shown) of the FL1, FL2 layers to be aligned in an oppositedirection to the magnetic moment of a third free layer (FL3) 33. Similarto NM1 functionality, the NM2 layer may be employed to balance dipolarfield and writing symmetry in the MTJ.

In other embodiments, NM2 layer 32 is a moment diluting layer with acomposition that is an element M selected from Ru, Ta, Ti, W, Zr, Hf,Nb, Mo, V, Mg, and Cr, or is an alloy which includes a magnetic element(Fe, Co, or Ni) and a non-magnetic element M as described earlier withregard to NM1 12. As a result, the magnetic moments of layers 30, 31, 33are aligned in the same direction and crystalline character in thesubsequently deposited FL3 layer is blocked until an annealing step isperformed following deposition of all MTJ layers. The thickness of theNM2 layer may vary between 1 and 10 Angstroms to adjust the couplingstrength between FL2 31 and FL3 33, the TMR ratio, and magnetostriction(λ). A strong coupling (Hcp) between the FL2 and FL3 layers is desirablein order to minimize noise in the MTJ and improve the signal to noise(SNR) ratio. Moreover, magnetic stability improves as Hcp increases. FL3layer 33 may be comprised of any magnetic material including Co, Fe,CoFe, and alloys thereof with Ni, B, or other metals. The FL3 layer maybe a laminate of Co or CoFe, with Ni or NiCo. The thickness of theFL1/FL2 stack is preferably less than or equal to about 20 Angstroms topromote PMA in the FL1 and FL2 layers.

In yet another embodiment (not shown), the NM2 layer may be omitted togive a free layer stack that is a trilayer represented by a FL1/FL2/FL3configuration.

Note that depending on the magnetic memory state “0” or “1” in MTJ 1,the magnetic moments (not shown) of FL1 30 and FL2 31 may be alignedeither in the same direction or in the opposite direction with respectto magnetic moment 14 a in FIG. 1a , or with respect to magnetic moment14 b in FIG. 1b . Moreover, the magnetization in the FL1 and FL2 layersis always in the same direction.

In all embodiments, the uppermost layer in the MTJ is a capping layer 40that may be Ru, Ta, or a combination thereof. In other embodiments, thecapping layer may comprise a metal oxide that interfaces with the freelayer stack 35-1 to promote or enhance PMA in the adjoining free layer.

In FIG. 2a , another embodiment of the present disclosure is depictedwhere the MTJ 2 has a top spin valve configuration. The composition oflayers is retained from FIG. 1a but the ordering of layers in the MTJstack is modified. In particular, free layer stack 35-1 is switched withreference layer 15 a 1, and the ordering of layers within each stack 15a 1, 35-1 is reversed such that FL3 now contacts the bottom layer 10,and AP1 layer 14 contacts a top surface of tunnel barrier 20. In otherwords, the free layer may have a FL2/FL1 structure or an optionalFL3/NM2/FL2/FL1 configuration with the FL1 layer contacting a bottomsurface of the tunnel barrier while the reference layer has anAP1/NM1/AP2 configuration in which a lower AP1 layer 14 has a boroncontent of 1-20 atomic % and an upper AP1 layer 13 has a boron contentof 25-50 atomic %. When a third AP1 layer 16 is included, layer 16 isthe uppermost AP1 layer. In some embodiments, an optional AFM layer 34is provided between AP2 layer 11 and capping layer 40. FIG. 2a indicatesreference layers 11, 13, 14 have in-plane magnetization 11 a, 13 a, 14a.

In a preferred top spin valve configuration illustrated in FIG. 2b ,each of the AP1 layers 13, 14, and AP2 layer 11 has PMA as indicated bymagnetic moment directions 13 b, 14 b, 11 b, respectively. Similar toFIG. 2a , the free layer may have a FL3/FL2/NM2/FL1 configuration whereFL3 33 contacts bottom layer 10, and FL1 30 contacts a bottom surface oftunnel barrier 20. Furthermore, AP1 layer 14 adjoins a top surface ofthe tunnel barrier, and the AP2 layer contacts the capping layer 40.When NM2 is a moment diluting layer, the magnetic moments (not shown) ofthe FL1, FL2, FL3 layers may either be aligned in the same direction asAP1 magnetization 14 b, or all may be aligned opposite to the AP1 layermagnetization 13 b, 14 b direction depending on the memory state of MTJ2. Optional AP1 layer 16 has a magnetization in the same direction asthe other AP1 layers.

In an alternative bottom spin valve embodiment depicted as MTJ 3 in FIG.3, the positions of FL1 30 and FL2 31 in FIG. 1b may be switched so thatthe FL2 layer contacts a top surface of the tunnel barrier 20 while FL1adjoins the NM2 layer 32 to give a FL2/FL1/NM2/FL3 configuration forfree layer 35-2. In alternative embodiments, the NM2 layer may beomitted to give a FL2/FL1/FL3 configuration, or both NM2 and FL3 areomitted to provide a FL2/FL1 free layer stack. Furthermore, AP1 layers13, 14 may be switched in the reference layer stack 15 b 2 to give acomposite AP1 layer 18 wherein AP1 layer 13 with 25-50 atomic % boroncontacts a bottom surface of the tunnel barrier, and AP1 layer 14adjoins a top surface of NM1 layer 12. When optional AP1 layer 16 isinserted in the AP1 stack 18, layer 16 is located a greater distancefrom the tunnel barrier than layers 13, 14. The reference layer 18 hasan AP2 layer 11 formed on the bottom layer 10 and NM1 is ananti-ferromagnetic coupling layer, or a moment diluting layer. In theexemplary embodiment representing a SyAP configuration withperpendicular magnetic anisotropy, the AP2 layer has a magnetic moment11 b aligned opposite to the direction of magnetic moments 13 b, 14 bfor the AP1 layers, and NM1 is an anti-ferromagnetic coupling layer.

Referring to FIG. 4, an alternative top spin valve embodiment is shownin MTJ 4 wherein the positions of free layer 35-2 and reference layer 15b 2 in FIG. 3 are switched. In addition, the ordering of layers withinthe free layer and reference layer are reversed. In particular, AP1layer 13 contacts a top surface of tunnel barrier 20 and AP1 layer 14 isformed on the AP1 layer with high B content while AP2 layer 11interfaces with a bottom surface of capping layer 40. Also, FL2 31contacts a bottom surface of the tunnel barrier, and FL3 33 interfaceswith a top surface of bottom layer 10. Otherwise, all the properties andcompositions of each of the layers in MTJ 3 and 4 are retained fromprevious embodiments. The NM2 layer 32 when present may either enableanti-ferromagnetic coupling between FL1 and FL3 layers in FIGS. 3-4, orprovide a moment diluting effect within the free layer.

In yet another embodiment depicted in FIG. 5, AP1 layer 17 in the FIG.1b embodiment is replaced by AP1 layer 18 to give a reference layer 15 b2 configuration as depicted earlier. Meanwhile, the free layer stack35-1 is retained from FIG. 1b such that high boron content FL1 layer 30contacts a top surface of the tunnel barrier 20. High boron content AP1layer 13 adjoins a bottom surface of the tunnel barrier. The presentdisclosure also anticipates a top spin valve design (not shown) whereplacement of free layer stack 35-1 and reference layer 15 b 2 isswitched.

Referring to FIG. 6, another embodiment of the present disclosure isillustrated where the free layer stack 35-1 in FIG. 1b is replaced byfree layer stack 35-2. Meanwhile, reference layer 15 b 1 is retainedsuch that low boron content AP1 layer 14 contacts a bottom surface oftunnel barrier 20. Low boron content FL2 31 adjoins a top surface of thetunnel barrier. It should be understood that the positions of referencelayer 15 b 1 and free layer stack 35-2 may be switched to provide a topspin valve version (not shown) of the MTJ stack found in FIG. 6.

The present disclosure also encompasses a method of fabricating a MTJ ina magnetic memory element as illustrated in FIGS. 7-9. According to oneembodiment, a MTJ stack of layers is formed on a bottom conductor 8depicted in FIG. 7. All layers in the MTJ stack may be deposited in a DCsputtering chamber of a sputtering system such as an Anelva C-7100sputter deposition system that includes ultra high vacuum DC magnetronsputter chambers with multiple targets and at least one oxidationchamber. Typically, the sputter deposition process involves an inert gassuch as Ar and a base pressure between 5×10⁻⁸ and 5×10⁻⁹ torr. A lowerpressure enables more uniform films to be deposited. The temperature inthe sputter deposition chamber during deposition processes may vary fromroom temperature to 400° C.

The fabrication process according to one embodiment involves depositinga seed layer and then a reference layer 15 b 1 or 15 b 2 as previouslydescribed. A first Mg, metal, or alloy layer having a thickness between4 and 8 Angstroms is deposited on an uppermost AP1 layer which is layer13 or 14 in a bottom spin valve embodiment, or on FL1 30 or FL2 31 in atop spin valve structure. Thereafter, the fabrication sequence involvesoxidizing the first Mg, metal, or alloy layer with a natural oxidation(NOX) process, and then depositing a second Mg, metal, or alloy layerwith a thickness of 2 to 4 Angstroms on the oxidized first Mg, metal, oralloy layer. The second Mg (or metal or alloy) layer serves to protectthe subsequently deposited free layer from oxidation. In a bottom spinvalve embodiment, the free layer stack is deposited followed by thecapping layer. During an annealing step that follows deposition of theuppermost layer in the MTJ stack of layers, oxygen tends to diffuse fromthe lower metal oxide layer into the second metal or alloy layer therebyoxidizing the latter to form a tunnel barrier that is substantiallyoxidized throughout.

The NOX process may be performed in an oxidation chamber within thesputter deposition system by applying an oxygen pressure of 0.1 mTorr to1 Torr for about 15 to 300 seconds. Oxygen pressure between 10⁻⁶ and 1Torr is preferred for an oxidation time mentioned above when aresistance×area (RA) value is desired from about 0.5 to 5 ohm-um². Amixture of 02 with other inert gases such as Ar, Kr, or Xe may also beused for better control of the oxidation process. In alternativeembodiments, the process to form a metal oxide or metal oxynitridetunnel barrier may comprise one or both of a natural oxidation and aconventional radical oxidation (ROX) process as appreciated by thoseskilled in the art.

Once all layers in the MTJ stack are formed, the MTJ stack is annealedin a vacuum oven between 330° C. to about 400° C. for about 1 to 5 hoursto enhance PMA in one or both of the reference layer and free layer,increase coercivity (Hc) and the uniaxial magnetic anisotropy field(Hk), and promote crystallinity in the AP1 layer/tunnel barrier/FL1/FL2stack of layers.

Next, a photoresist layer is coated on a top surface of the MTJ stackand is then patternwise exposed and developed to provide a photoresistmask 55. Thereafter, a conventional ion beam etch (IBE) or reactive ionetch (RIE) process is employed to remove unprotected portions of the MTJstack and generate MTJ element 1 with sidewalls 1 s that extend to a topsurface 8 t of the bottom conductor. The sidewalls may be perpendicularto the bottom conductor top surface, but are often non-vertical becauseof the nature of the etching process employed for the sidewall formationprocess. Openings 40 are formed on each side of the MTJ element. Itshould be understood that the photoresist patterning and etchingsequence forms a plurality of MTJ elements typically arrayed in rows andcolumns on a plurality of bottom conductors. However, only one MTJ andone bottom conductor are shown in order to simplify the drawing.

Referring to FIG. 8, a first insulation layer 50 is deposited alongsidewalls 1 s and fills openings 40 between adjacent MTJ elements.Thereafter, a well known chemical mechanical polish (CMP) process may beperformed to remove the photoresist mask and form a top surface 50 t onthe insulation layer that is coplanar with a top surface 1 t of MTJ 1.

Referring to FIG. 9, a conventional sequence of steps that includesphotoresist patterning and etch processes is used to form a topconductor 60 within a second insulation layer 70 wherein the topconductor adjoins the top surface of MTJ 1. The top conductor processtypically produces a plurality of top conductor lines formed in aparallel array but only one top conductor is shown to simplify thedrawing.

An experiment was conducted to demonstrate the improved performanceachieved by implementing a reference layer/tunnel barrier/free layerstack in a MTJ according to an embodiment of the present disclosure. TwoMTJ elements hereafter referred to as MTJ A and B and shown in Table 1were fabricated with a seed layer/AP2/NM1/AP1/MgO/FL1/FL2/capping layerconfiguration. The key difference is that MTJ A includes a high boroncontent alloy (Fe₇₀B₃₀) in both of the AP1 layer and FL1 layer accordingto an embodiment of the present disclosure while MTJ B is formedaccording to a process of record (POR) practiced by the inventors andhas the high boron content alloy only in FL1.

TABLE 1 Defect rate comparison for MTJ elements with AP1/MgO/FL1/FL2bottom spin valve configurations Defect Defect rate: rate: Free layer330° C., 400° C., AP1 layer (FL1/FL2) 30 min. 30 min. MTJ compositioncomposition anneal anneal A Co₂₀Fe₆₀B₂₀/Fe₇₀B₃₀ Fe₇₀B₃₀/Co₂₀Fe₆₀B₂₀ 10ppm 10 ppm B Co₂₀Fe₆₀B₂₀ Fe₇₀B₃₀/Co₂₀Fe₆₀B₂₀ 10 ppm 30 ppm

For each MTJ configuration (A and B) shown in Table 1, a MTJ stack oflayers was patterned into 100 nm circular devices. Defect rates wereobtained by measuring test chips containing 8 Mb (8,388,608) devices perchip. The results from hundreds of test chips were averaged to providethe data shown in Table 1. Although the defect rate of MTJ A and MTJ Bwere both 10 ppm after a 330° C. anneal for 30 minutes, we observed asignificant advantage with MTJ A following a 400° C. anneal since theMTJ A defect rate was maintained at 10 ppm. However, the defect rate forMTJ B increased threefold to 30 ppm after a 400° C., 30 minute annealingprocess.

It should be noted that the reference layer/tunnel barrier/free layerstack of the present disclosure may also be incorporated in magnetictunnel junction that is used as a sensor in a read head, for instance.In this case, the MTJ element is formed between a bottom shield and atop shield in the read head.

The magnetic layers disclosed in the embodiments found herein, and inparticular the boron containing alloys, may be fabricated withoutadditional cost since no new sputtering targets or sputter chambers arerequired. No change in process flow is needed in current manufacturingschemes in order to implement one or more magnetic layers with a boroncontent as high as 50 atomic %. It should also be understood that onemay also implement a MTJ formed according to an embodiment of thepresent disclosure in domain wall motion devices and in MRAM deviceshaving more than one MgO tunnel barrier such as those devices with twotunnel barriers, and three terminals.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

What is claimed is:
 1. A method of forming a magnetic tunnel junction(MTJ) element in a memory device, comprising: providing a substrate;forming a reference layer on the substrate, the reference layer has anAP2/NM1/AP1 configuration wherein AP2 is a first magnetic layer, AP1 isa second magnetic layer, and NM1 is a first non-magnetic layer thatenables anti-ferromagnetic coupling between AP1 and AP2, or provides amoment diluting effect in the reference layer, the AP1 layer comprises afirst layer with a boron content between 25 and 50 atomic %, and asecond layer with a boron content between 1 and 20 atomic %; forming thetunnel barrier on the AP1 layer, the tunnel barrier has a first surfacethat contacts either the first layer or the second layer in the AP1layer; forming a free magnetic layer stack on the tunnel barrier, thefree magnetic layer stack is comprised of a first free magnetic layerwith a boron content between 25 and 50 atomic %, and a second freemagnetic layer with a boron content between 1 and 20 atomic % whereinthe first free magnetic layer forms an interface with the second freemagnetic layer and one of the first or second free magnetic layerscontacts the tunnel barrier at a surface opposite to the first surface;and performing an anneal process at a temperature of about 400° C. 2.The method of claim 1, wherein the first layer in the AP1 layerphysically contacts the tunnel barrier, and the second layer in the AP1layer adjoins a surface of the first layer that is opposite the firstsurface.
 3. The method of claim 1, wherein the second layer in the AP1layer physically contacts the tunnel barrier, and the first layer in theAP1 layer adjoins a surface of the second layer that is opposite thefirst surface.
 4. The method of claim 1, wherein each of the first andsecond layers in the AP1 layer, and FL1, and FL2 have a composition thatis selected from CoB, FeB, CoFeB, CoFeNiB, or CoFeBQ wherein Q is one ofZr, Hf, Nb, Ta, Mo, and W.
 5. The method of claim 1, wherein at leastone of the first and second layers of the AP1 layer is amorphous uponformation on the substrate.
 6. The method of claim 1, wherein the AP1layer further includes a third layer having ferromagnetic propertieslocated between the first layer and the NM1 layer.
 7. The method ofclaim 1, wherein one or both of the first and second layers in the AP1layer have a multilayer configuration comprising one or more B layersand one or more layers that include a material selected from the groupconsisting of Co, Fe, CoFe, CoFeNi, CoFeQ, and CoFeB, and wherein Q isone of Zr, Hf, Nb, Ta, Mo, and W.
 8. The method of claim 1, wherein oneor both of the first and second free magnetic layers have a multilayerconfiguration comprised of one or more layers of B, and one or morelayers that include a material selected from the group consisting of Co,Fe, CoFe, CoFeNi, CoFeQ, and CoFeB, and wherein Q is one of Zr, Hf, Nb,Ta, Mo, and W.
 9. The method of claim 1, wherein NM1 is one of Ru, Rh,and Ir to give a synthetic anti-parallel (SyAP) configuration for thereference layer.
 10. The method of claim 1, wherein the free magneticlayer stack has a FL1/FL2/NM2/FL3 configuration wherein FL1 is one ofthe first or second free magnetic layers, FL2 is the other of the firstor second free magnetic layers, FL3 is a third free magnetic layer, andNM2 is a second non-magnetic layer that enables anti-ferromagneticcoupling between FL2 and FL3, or provides a moment diluting effect inthe free magnetic layer stack.
 11. A method comprising: forming areference layer on a substrate, the reference layer having anAP2/NM1/AP1 configuration wherein AP2 is a first magnetic layer, AP1 isa second magnetic layer, and NM1 is a first non-magnetic layer, whereinthe AP1 layer includes a first layer with a boron content between 25 and50 atomic % and a second layer with a boron content between 1 and 20atomic %, wherein at least one of the first and second layers of the AP1layer is amorphous upon formation on the substrate; forming a tunnelbarrier layer on the AP1 layer; forming a free magnetic layer stack onthe tunnel barrier, the free magnetic layer stack is comprised of afirst free magnetic layer with a boron content between 25 and 50 atomic%, and a second free magnetic layer with a boron content between 1 and20 atomic %, wherein the first free magnetic layer interfaces with thesecond free magnetic layer; and after forming the free magnetic layerstack on the tunnel barrier, performing an anneal process.
 12. Themethod of claim 11, wherein the NM1 layer is amorphous upon formation onthe substrate.
 13. The method of claim 11, wherein the tunnel barrierlayer interfaces with the second layer from the AP1 layer and interfaceswith the first free magnetic layer from the free magnetic layer stack.14. The method of claim 11, wherein the tunnel barrier layer interfaceswith the first layer from the AP1 layer and interfaces with the secondfree magnetic layer from the free magnetic layer stack.
 15. The methodof claim 11, wherein the tunnel barrier layer interfaces with the firstlayer from the AP1 layer and interfaces with the first free magneticlayer from the free magnetic layer stack.
 16. The method of claim 11,wherein the tunnel barrier layer interfaces with the second layer fromthe AP1 layer and interfaces with the second free magnetic layer fromthe free magnetic layer stack.
 17. A method comprising: forming a freemagnetic layer stack on a substrate, the free magnetic layer stack iscomprised of a first free magnetic layer with a boron content between 25and 50 atomic %, and a second free magnetic layer with a boron contentbetween 1 and 20 atomic %, wherein at least one of the first freemagnetic layer and the second free magnetic layer is amorphous uponformation on the substrate; forming a tunnel barrier layer on the freemagnetic layer stack; forming a reference layer on the tunnel barrierlayer, the reference layer having an AP2/NM1/AP1 configuration whereinAP2 is a first magnetic layer, AP1 is a second magnetic layer, and NM1is a first non-magnetic layer, wherein the AP1 layer includes a firstlayer with a boron content between 25 and 50 atomic % and a second layerwith a boron content between 1 and 20 atomic %; and after forming thereference layer on the tunnel barrier, performing an anneal process. 18.The method of claim 17, wherein the tunnel barrier layer interfaces withthe second layer from the AP1 layer and interfaces with the first freemagnetic layer from the free magnetic layer stack.
 19. The method ofclaim 17, wherein the tunnel barrier layer interfaces with the firstlayer from the AP1 layer and interfaces with the second free magneticlayer from the free magnetic layer stack.
 20. The method of claim 17,wherein the first and second layers of the AP1 layer each have amagnetic moment in a first direction, wherein the AP2 layer has amagnetic moment in a second direction opposite the first direction.